Abstract
Aurihydrargyrumite, a natural Au6Hg5 phase, was found in Iyoki, Uchiko, Ehime Prefecture, Shikoku Island, Japan. Aurihydrargyrumite with a metallic silver luster occurs as a submicron- to 2 μm-thick layer on the outermost surface of the placer gold. A prismatic face may be formed by {001} and {100} or {110}. The streak is also silver white and its Mohs hardness value is ca. 2.5. Its tenacity is ductile and malleable, and its density, as calculated based on the empirical formula and powder unit-cell data, is 16.86 g·cm−3. The empirical formula of aurihydrargyrumite, on the basis of 11 Au + Hg, is Au5.95Hg5.05. Aurihydrargyrumite is hexagonal, P63/mcm, with the lattice parameters a = 6.9960(10) Å, c = 10.154(2) Å and V = 430.40(15) Å3, which is identical with the synthetic Au6Hg5 phase. The seven strongest lines in the powder X-ray diffraction (XRD) pattern [d in Å(I/I0)(hkl)] were 2.877(29)(112), 2.434(42)(113), 2.337(100)(104), 2.234(87)(211), 1.401(39)(314), 1.301(41)(404), and 1.225(65)(217). Aurihydrargyrumite forms through the weathering of mercury-bearing placer gold by involvement of self-electrorefining. This new mineral has been approved by the IMA-CNMNC (2017-003) and it is named for its composition, being a natural amalgam of gold (Latin: aurum) and mercury (Latin: hydrargyrum).
Highlights
The investigation of the phase relations in the Au–Hg system has a long history
The X-ray diffraction (XRD) pattern of this amalgam was indexed as monoclinic symmetry, this phase does not appear in the phase diagram
Barkov et al [7] reported zoned amalgam of composition (Au1.5-1.9 Ag1.1-1.4 )Σ2.8-3.0 Hg1.0-1.2 from a placer deposit in the Tulameen–Similkameen river system, British Columbia, Canada. They discussed the concentration of the mercury component at the grain rim by some electrochemical factors that are related to the process of self-electrorefining, they considered the amalgam grain to be anthropogenic in origin [7]
Summary
The investigation of the phase relations in the Au–Hg system has a long history. In the latest phase diagram [1], three amalgam solid phases appear in the system: Au4 Hg, Au3 Hg, and Au2 Hg. Several amalgam phases without structural identification have been reported from the placer deposit from the Palakharya River, Bulgaria [3,4]. Barkov et al [7] reported zoned amalgam of composition (Au1.5-1.9 Ag1.1-1.4 )Σ2.8-3.0 Hg1.0-1.2 from a placer deposit in the Tulameen–Similkameen river system, British Columbia, Canada They discussed the concentration of the mercury component at the grain rim by some electrochemical factors that are related to the process of self-electrorefining, they considered the amalgam grain to be anthropogenic in origin [7]. The new mineral, which we name aurihydrargyrumite, is named after its chemical composition, being a natural amalgam of gold (Latin: aurum) and mercury (Latin: hydrargyrum).
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